Buffs Needed:
Bard Songs
Prayer
Haste
Rage
Sonic Resist (Orthon's)
Fire Resist
Fire Protection
Poison Resist
Freedom Of Movement (Sleet Storm)
Blur
Displacement
Greater Heroism
Magic Circle Against Evil
Shield of Faith Mass
Mass Aid

Crowd Control Spells:
Dancing Ball - On Rally Point
Symbol of Stunning - On Rally Point
Solid Fog - On Rally Point

Offensive Spells:
Cone of Cold - On Pit Fiend (Manage your mana, you are on Gnoll Duty)

Weapons To Use:
Vorpal - Mob Beater
Transmuting of Pure Good
Transmuting Silver
Silver of Pure Good
Transmuting Silver of Pure Good

Scrolls/Pots:
Fire Protection
Major Mneumonics
Heal Scrolls

Everyone should really be carrying Fire Pro Pots.  They help your healers out by mitigating damage you may receive from a meteor to the face.

Tips:
The Pit Fiend on Normal has DR 15 against Good, and DR 15 against Silver, with Light Fortification.  You must bypass both DR's or you will get Yellow Damage numbers.

If you die, you will be put in a penalty box, so just sit back, relax, and pray your group can get it done without you.

Assignments:
Healer/Tank pairs (If using Healing Strat 2)
Caster for Haste
Portal Assignments for Gnolls

Healing Strategy 1:
Mass Healing.  When tanks are on the pit fiend, each healer will take a turn at doing Mass Healing buffs on one main tank, the rest of the people hitting the pit fiend should be on top of this tank to get the mass heals.  This strategy is a bit trickier than the "hero" method, and takes some getting used to.  A good rule of thumb, if you have 3 healers, you must be able to take the pit fiend down in 3 rounds, 4 healers, 4 rounds, etc., for this strategy to truly be successful.  If your rounds go over the number of healers you have, you will run out of SP.  2 Healers can do it in 3 round, over that you'll be drinking pots.

Healing Strategy 2:  (Nearly Obsolete, only used if you have no DPS)
Hero Method.  Each healer in the group gets assigned 1 tank.  If you feel you can handle it, take two.  As the tank gets on the pit fiend, this healer will concentrate only on their assigned tank, and keep them alive.  This is less hectic than method 1.  You can alternate between heal scrolls and your SP to conserve some of it.  I tend to use strictly heal scrolls, and save my SP for mass healing in between rounds when taking on the Devils.

Gnoll Strategy:
Caster should be handling all Gnolls, healers/DPS should be concentrating on Aertrikos.

General Strategy (Applies to both healing strategies below):
Pull out your Vorpals!!  Trash mobs should always be Vorpaled.  When you zone in, lay down a Dancing Ball, Symbol of Stunning, and Solid Fog if you have it.  Healers should move off from the main group, a bit more to the south west, and should not be in the middle of the battle.  Shield block in case a devil or orthon pops on you.  If they do stand still, they will port back to the main group shortly.  Use Mass Healing on the main group here.  Have one person head to the center and pull all the mobs back to the Rally Point (SW).  After defeating all of these mobs, 4 Devils will spawn out of the portals.  Only 1 healer should be healing through this, so you are not double dipping the mana pool.

WARNING - As you kill these devils, they will turn into whirling blades, so be careful not to take to much damage.  After killing the final Devil, the Pit Fiend will appear in the center.  

Pull out your Pit Fiend Weapons and get on him.  Casters should get an Extended Haste on the main tank group.  Whatever healing strategy you are using, implement it now.  Tanks should all come from one side of the Fiend, I find North to be a good side, any will do.  Mass heals will not miss you this way.  Anyone that is not a healer, or main tank, should be moving around to avoid the meteors that will occasionally come from the Fiend, and Range Him.  Keep Fire Prot on yourself healers!

NOTE - Quicken is great for healers in here, if you get interrupted in healing, most likely your tank will die, quicken prevents interruptions in casting from your SP pool (not scrolls/wands).

As time goes on, blades will be whirling around the outer edge, moving closer and closer to the center where the Pit Fiend is.  When they get precariously close to you the tanks, its up to the healers to call whether they should remain in, or bounce out.

The Pit Fiend will at this point disappear, and fly N to S, then E to W.  RALLY POINT TIME!!  Get to the SW corner with the rest of your group, heal everyone up, get Fire Protection cast on the tanks again (Mass Protect).  Get a Dancing Ball up for the next round of Devils.  Every time the Pit Fiend goes up, after his two passes, 4 more devils will spawn, again as they die, whirling blades will take the place of their dead bodies so be careful.  Once the 4 Devils are down, its GNOLL TIME.  Casters get ready.  Two Gnolls will pop at each portal.  Kill them as fast as you can.  Tanks should already be on the Fiend at this point, while casters are taking down the gnolls.

Rinse and repeat until the fiend is down.  2-3 Rounds on normal difficulty, is the norm, if you are going more rounds than that, you are expending to many resources.

WARNING - Once the fiend is defeated, make sure everyone is ready to move on to the next part before attempting to head through the portal, the first person through, kills everyone in your group, and you all get ported into the next round.  The death is inevitable, but if you have people AFK when you do this, you will fail the next part miserably.

WARNING2 - If you die in this part, you will not come back until part 5, this is a MOD8 Patch 2 update.  You will need to use the portals at the end, after completing, to get your part 4 chest.

NOTE - You can REZ the person that died.  If you click the alter, and have a cleric waiting at the zone in point, and they con the dead person and send the rez immediately, and the person takes it immediately, they can get there chests still without having to go back.